Camshaft sensors are used in motor vehicles to determine the positions of the various cylinders in the combustion cycle of the engine; in other words, to determine whether the intake stroke, the compression stroke, the explosion stroke or the exhaust stroke is taking place in each cylinder. Sensors of this type include a magnetic field generator (such as a permanent magnet), a magnetic field detection means (for example a Hall effect cell, a magnetoresistive (MR) cell, a giant magnetoresistive (GMR) cell, or other type), and an electronic circuit for processing the signal received by the magnetic field detection means. These sensors, called active sensors, deliver a digital signal to a central computer for processing.
The magnetic field generator may also be the target, made of a magnetic material and having alternating south and north poles. In this case, the sensor may or may not incorporate a permanent magnet, depending on the detection means used. In the following text, the south and north poles are considered to be equivalent to the teeth and intervals of a mechanical target.
A camshaft sensor is associated in a known way with a target fixed to a camshaft. This target takes the form of a disk having a toothed periphery. These teeth all have the same height, but different spacings (intervals) and lengths, thereby providing a means (known in itself) of encoding the positioning of the cylinders in the combustion cycle of an internal combustion engine for a motor vehicle.
The magnetic field detection means present in the sensor detects the passage of the teeth of the target in front of the sensor, and the resulting signal can be used to determine the position of each cylinder with respect to the combustion cycle of the engine, in a known way.
The position of each cylinder in the engine cycle is determined by observing the curve of variations of the magnetic field sensed by the camshaft sensor during one revolution of the target. This curve has a succession of peaks, each corresponding to one tooth of the target. By measuring the spacing between each peak and the next, and the duration of each of them, the position of each cylinder can be determined with respect to the engine combustion cycle. For this purpose, it is therefore important to ensure the accuracy of the position of the electrical edges of the signal generated by the sensor relative to the position of the mechanical edges of the target. Each of its electrical edges represents the passage of the target mechanical edges. The aim is to minimize the phase shift of the signal caused by the variable spacing between the sensor and target. The electrical signal generated by the sensor changes state (high or low) when the magnetic signal crosses a predetermined threshold proportional to its amplitude. For this purpose, this threshold is set (at 75%, corresponding to an optimum level in respect of the accuracy of the electrical edges relative to the mechanical edges for most existing targets) in order to determine the instant of passage of each edge defining a tooth. Thus, as soon as a first maximum and a first minimum of the sensed magnetic field are detected, the threshold value corresponding to 75% of this amplitude is identified, and it is assumed that a falling edge is detected if the measured value of the magnetic field falls below this threshold value, and conversely a rising edge is detected if the measured value of the magnetic field rises above this threshold value (or vice versa). By this means, the moment of detection of the edge is optimized. However, this method presupposes that all the teeth have the same height and that there is no geometrical discrepancy between the various teeth. It therefore presupposes that the geometry of the target is virtually perfect.
However, these systems (of sensors and target) have the drawback of being sensitive to the positioning of the target on the camshaft and to the geometry of this target.
For reasons of cost, the targets, which are simple metal components having teeth of predetermined sizes at predetermined spacings, are serially produced and often have an imperfect geometry. Notably, the teeth do not always have the same height with respect to the center of the target. This defect is called “out-of-roundness”. The result of this is that the upper parts of the target teeth are not all located on the same circle centered on the camshaft. This problem is therefore termed “out-of-roundness”. This out-of-roundness of the target manufacturing may be accompanied by an out-of-roundness due to the assembly of the target on the camshaft.
Clearly, since the camshaft sensor measures the variations of the magnetic field created by the passage of the teeth in front of it, if a tooth is lower (or higher) than the others, the spacing between this tooth and the sensor varies relative to the other teeth and causes a variation of the sensed magnetic field. These variations of magnetic fields may falsify the measurements made (by decreasing the accuracy of the position of the electrical edges relative to the mechanical edges), or may even fail to be interpreted by the sensor (where a tooth is undetected because the magnetic field is below the detection threshold). The signal delivered by the camshaft sensor is then erroneous, and the determination of the position of each cylinder in the engine cycle is distorted, or may even be impossible to carry out correctly.
US 2009/0001965 describes a magnetic field sensor and a method for calibrating this sensor in which the variations of the magnetic field caused by the rotation of a target are continuously measured in order to determine the received maximum and minimum magnetic fields values. However, this method cannot compensate for incorrect positioning (or for an incorrect geometry of the target), since the calculation of the switching threshold values does not detect these anomalies.
For its part, U.S. Pat. No. 6,967,477 describes an auto-adaptive toothed wheel sensor. However, this sensor does not detect errors in the positioning or geometry of toothed wheels.